In existing inks for inkjet recording, those of a water-soluble dye of every sort dissolved in an aqueous medium is generally used. Recently, an ink of a pigment dispersed in an aqueous medium is also proposed. This is because the ink using a pigment is excellent in water resistance and light fastness as compared with the ink using a water-soluble dye.
When a pigment is dispersed in a highly volatile solvent such as water to prepare an inkjet ink, a discharge performance from an inkjet head must be taken into consideration. When the solvent component volatilizes, a balance in a blending ratio between the pigment component and the solvent component is lost, and a concentration of the pigment in the ink in the vicinity of a nozzle extremely increases. As a result, a phenomenon such as an increase of viscosity of the ink or deposition of a coarse particle is caused. The nozzle is clogged, resulting in a lowering of the discharge performance. Specifically, a prescribed discharge volume is not obtainable, and the flying direction becomes inaccurate. According to circumstances, a misfiring nozzle appears, too.
Japanese Patent No. 3050049 proposes an ink containing, as a water-soluble solvent, glycerin and propylene glycol. Though the discharge performance is improved by the water-soluble solvent, there is still room for improvements in print quality regarding the print image density.
In the majority of existing pigment inks, by chiefly suppressing penetration properties, wetting of the ink against the paper surface is suppressed, thereby storing ink droplets in the vicinity of the paper surface to secure the print quality. Inks of such a kind are not always able to adapt to all kinds of paper, and a difference in bleeding due to a difference of the paper kind is large. In particular, a stable print quality is not obtainable in regenerated paper. The regenerated paper is manufactured by mixing an unspecified large number of kinds of papers. The difference in wetting characteristics of the ink against the kind of paper to be used affects the generation of bleeding. Also, according to existing pigment inks, it takes a long period of time to dry a printed area to be caused due to a composition thereof. In particular, in multicolor printing such as color printing, colors which are adjacent to each other cause color mixing. Furthermore, there is a problem that since the pigment remains on the surface of paper or the like, scratch resistance becomes worse.
For example, JP-A-9-111165 and U.S. Pat. No. 5,156,675 propose that a glycol ether based penetrating agent is blended in an ink, thereby enhancing penetration properties of the ink into paper. For the purpose of dispersing a pigment in an aqueous medium, a surfactant or a dispersant such as a polymer dispersant is contained in an existing pigment ink. There is a possibility that the glycol ether based penetrating agent impairs effects of the dispersant. There was involved a problem that when a large quantity of the glycol ether based penetrating agent is added, the dispersion stability of the pigment is lowered with a lapse of time.
It is the present situation that an ink which is an aqueous ink for inkjet recording which is adaptive to recording on plain paper and which is excellent in both quick drying properties and image quality of a printed matter and free from clogging by the ink in intermittent discharge and in which a pigment is stably dispersed even with a lapse of time is not obtained yet.